1. Field
Aspects of the present disclosure relate generally to wireless communication systems, and more particularly, to enabling softer handover by user equipment in a wireless communication system.
2. Background
Wireless communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such networks, which are usually multiple access networks, support communications for multiple users by sharing the available network resources. One example of such a network is the UMTS Terrestrial Radio Access Network (UTRAN). The UTRAN is the radio access network (RAN) defined as a part of the Universal Mobile Telecommunications System (UMTS), a third generation (3G) mobile phone technology supported by the 3rd Generation Partnership Project (3GPP). UMTS, which is the successor to Global System for Mobile Communications (GSM) technologies, currently supports various air interface standards, such as Wideband-Code Division Multiple Access (W-CDMA), Time Division-Code Division Multiple Access (TD-CDMA), and Time Division-Synchronous Code Division Multiple Access (TD-SCDMA). UMTS also supports enhanced 3G data communications protocols, such as High Speed Packet Access (HSPA), which provides higher data transfer speeds and capacity to associated UMTS networks.
“Softer handover” or “softer handoff” refers to a mode of operation used by the UMTS standards, where a mobile station is simultaneously connected to two or more sectors of the same physical cell site (e.g., cell) during a call, and thus, its active set size is greater than one. If the mobile station is simultaneously connected to two or more different physical cell sites then the operation is referred to as “soft handover” or “soft handoff.” Soft and/or softer handover are forms of mobile station-assisted handover. Due to the properties of the CDMA signaling scheme, it is possible for a CDMA mobile station to simultaneously receive signals from two or more remote radio heads (RRHs) and/or radio base stations that are transmitting the same bit stream (although perhaps using different transmission codes) on different physical channels in the same frequency bandwidth. If the signal power from two or more RRHs is nearly the same, the phone receiver can combine the received signals in such a way that the bit stream is decoded much more reliably than if only one RRH were transmitting to the mobile station. If any one of these signals fades significantly, the mobile station may have adequate signal strength from one of the other RRHs. Thus, soft and softer handover mitigate interference and provide improved quality of service.
Furthermore, in many modern wireless communication systems, mobile stations may assume any of various different states based on their needs at any particular time to enable a high level of control over power usage. For example, these states can include: states with dedicated resources assigned to the mobile station; various levels of standby states having corresponding tiers of communication capabilities; and idle states with little to no wireless connectivity. Within the various standby states, the network may have a reduced level of control over the plurality of mobile stations throughout the cell.
In a particular example, such as a conventional 3GPP UMTS network, one of the standby states is referred to as Cell_FACH. According to current specifications, the network is limited in that the mobile stations cannot be in soft and/or softer handover mode when the mobile stations are operating in the Cell_FACH state, and thus, the mobile stations are limited to having an active set size equal to or less than one while in the Cell_FACH state. This limitation currently persists notwithstanding the availability of softer handover in dedicated resource channel states, such as Cell_DCH.
Distributed base station systems with remote radio head capability allow for increased signal coverage area in a cost effective manner. Such systems feature multiple RRHs all connected (e.g., through fiber optic cables) to a central baseband processing unit associated with the base station to which they are connected. The RRHs may be deployed to provide extended signal coverage to sectors associated with different geographical areas. In some cases, the RRHs may be deployed in such a manner so that they provide increased and/or overlapping signal coverage to geographical areas that already receive coverage at least in part from one or more other RRHs, and are thus already associated with one or more sectors. As the deployment of RRHs continues to increase in UMTS and other networks, the proportion of mobile stations that fall within the overlapping coverage area of two adjacent sectors belonging to the same base station increases accordingly. Thus, there is a need to provide softer handover operation in CELL_FACH.
As the demand for mobile broadband access continues to increase, research and development continue to advance the UMTS technologies not only to meet the growing demand for mobile broadband access, but to advance and enhance the user experience with mobile communications.